Bob becomes Alice's boyfriend and she snaps from it.
The breakup makes Alice reconsider the aspects of her and Bob's personalities that caused it, and she becomes a much better person (while Bob remains much the same).
Subverted: Bob thinks Alice is psycho, but it turns out that he is simply taking her innocent behavior out of context.
Double Subverted: Alice's actions seem to have simply been misunderstood. And then it is revealed that she is psycho after all.
Parodied: Bob breaks up with Alice and assumes she is going to become this and becomes paranoid as a result. However Alice is in fact treating their break-up in a healthy well adjusted manner, but Bob continues to believe she is psycho.
Zig Zagged: Alice only freaks when she sees certain girls around Bob, but otherwise doesn't act aggressive towards him.
Averted: Alice doesn't snap when Bob ditches her.
Enforced: Alice was originally going to take the break up well, but the creators needed a new villain, and thus they made Alice the Psycho Ex-Girlfriend she is today.
Bob wants Alice to snap so he could have her target his enemies in return for being his girlfriend again.
Alice initially takes the breakup well enough, until her friends or family convince her that Bob was a Bastard Boyfriend and that she deserves revenge, or that she should intercede to "protect" his new girlfriend who obviously doesn't know what she's getting into.
Exploited: Alternatively, Alice realizes Bob's going to dump her sooner or later because the audience prefers Jane as a love interest for Bob. She seeks counseling or institutionalizes herself before it even happens. Or starts stockpiling weapons and planting tracking devices in Bob's clothes.
Bob never leaves Alice because he knows she will snap upon him ditching her.
Alice sees Bob's scared expression the first time they bump into each other after the breakup, and she's offended. "You think that just because I wanted us to be together, I'm going to have a mental breakdown when you leave? Get over yourself!"
Discussed: "Alice, if I were to leave you, what would your reaction be?" "Well, I will first find you and then I will kill you, just so you can never leave me again. Why do you ask?" "No reason..."
Deconstructed: Bob gets the courts to pass a restraining order on Alice upon finding out about this.
Reconstructed: This doesn't stop Alice from changing her identity and pursuing him.
Plotted A Good Waste: Alice is played as a run-of-the-mill Psycho Ex Girlfriend for most of the plot, and feminists everywhere grind their teeth. But hints begin to come to light that Bob was more than a Bastard Boyfriend; he systematically psychologically abused her into being utterly dependent on him, and destroyed her relationships with everyone else she loved. Then when he got tired of her "clinginess", he cast her aside and made her think it was her own fault. She's now convinced that the only way she can be happy is to convince him to take her back.